how will the capacitance change on placing a metallic slab in between the plates

capacitance will increase

suppose K is the dielectric constant of mettalic slab then new capacitance C is given by

C=K*C₀

WHERE C₀ IS THE CAPACITANCE WITHOUT INTRODUCING THE GLASS SLAB AND C IS THE CAPACITANCE AFTER INTRODUCING THE GLASS SLAB

It is not a glass slab it is a metallic slab. According to me the capacitance should increase. This is because the metallic slab will have charges induced on both its surfaces. Now apply the formula for capacitors in series and you'll get the result.